1. Field of the Invention
The present invention relates to an apparatus for adsorbing and removing hydrocarbons included in exhaust gas from an internal combustion engine during a cold period immediately after engine start during which a three-way or oxidation catalyst installed in the exhaust conduit has not yet been warmed-up and activated.
2. Description of the Prior Art
Hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) included in the exhaust gas from an internal combustion engine are purified usually by a three-way catalyst installed in an exhaust conduit of the internal combustion engine. However, during a cold period immediately after engine start, when the three-way or oxidation catalyst has not yet been warmed-up to temperatures above the activation temperature (in a range about 250.degree. C.-300.degree. C.), it cannot purify the HC of the cold period (cold HC) and CO included in the exhaust gas, though the cold HC occupies a main portion of the total amount of HC exhausted from the engine to atmosphere.
To prevent the cold HC from being exhausted to atmosphere, Japanese Patent Publication 2-135126 proposes a cold HC adsorption and removal apparatus, in which an HC adsorbent is installed upstream of the three-way or oxidation catalyst in the exhaust conduit of the internal combustion engine. The HC adsorbent includes zeolite coated onto a monolithic cordierite carrier and at least one kind of catalyst metal carried by the carrier. Since zeolite has a characteristic of adsorbing HC at relatively low temperatures (below about 90.degree. C.) and releasing the adsorbed HC at relatively high temperatures, HC included in the exhaust gas during a cold period is adsorbed by the HC adsorbent. After the exhaust gas temperature has risen and the three-way or oxidation catalyst has been activated, the HC released from the HC adsorbent flows to the three-way or oxidation catalyst located downstream of the HC adsorbent and is purified by the three-way or oxidation catalyst. The HC released from the HC adsorbent at temperatures between the HC release beginning temperature (about 90.degree. C.) of the HC adsorbent and the activation temperature (about 300.degree. C.) of the three-way catalyt is incompletely purified solely by the catalyst metal of the HC adsorbent.
Nevertheless, the HC purification efficiency of the conventional cold HC adsorption and removal apparatus is still relatively low. The reason appears to be that since the HC adsorbent is located upstream of the three-way catalyst, the temperature of the HC adsorbent rises quickly to the HC release beginning temperature (90.degree. C.) and begins to release the adsorbed HC before the three-way catalyst is warmed-up to 300.degree. C. to be activated, nor can the released HC be effectively purified by the catalyst metal of the HC adsorbent until the temperature of the catalyst metal of the HC adsorbent reaches about 300.degree. C.